This study attempts to explain explicitly the direct and quantitative effects of complicated urban built-environment on near-road dispersion and levels of vehicular emissions at the scale of several city blocks, based on ultrafine particle concentrations ([UFP]). On short timescales, ultrafine particles are an excellent proxy for other roadway emissions. Five measurement sites in the greater Los Angeles with different built environments but similar mesoscale meteorology were explored. After controlling for traffic, for most sampling days and sites, morning [UFP] were higher than those in the afternoon due to limited dispersion capacity combined with a relatively stable surface layer. [UFP] at the intersection corners were also higher than those over the sampling sites, implying that accelerating vehicles around the intersections contributed to [UFP] elevation. In the calm morning, the areal aspect ratio (Ararea), developed in this study for real urban configurations, showed a strong relationship with block-scale [UFP]. Ararea includes the building area-weighted building height, the amount of open space, and the building footprint. In the afternoon, however, when wind speeds were generally higher and turbulence was stronger, vertical turbulence intensity σw was the most effective factor controlling [UFP]. The surrounding built environment appears to play an indirect role in observed [UFP], by affecting surface level micrometeorology. The effects are substantial; controlling for traffic, differences in Ararea and building heterogeneity were related to differences in [UFP] of factors of two to three among our five study sites. These results have significant implications for pedestrian exposure as well as transit-oriented urban planning.
Mobile air pollution monitoring offers an opportunity to "map" pollutants with much higher spatial resolution than sparse stationary monitors. We develop a framework to address the challenges and constraints to developing higher spatial resolution maps from mobile data. The challenges include the non-uniform spatial resolution and distribution of the measurements; that measurements are made at slightly different locations in each pass of the mobile monitoring platform along a specific route (each "run"); in some cases, the poor precision of global positioning system coordinate data; potential for over/underweighting data; and varying urban background concentrations. We find that use of a reference grid and piecewise cubic Hermite spline interpolation between measurements to give equal weight to each sampling "run" at each grid reference point addresses many of the challenges effectively. A background correction was implemented to facilitate averaging over several sessions. For 1 s time resolution data collected at normal city driving speeds, we show that concentration maps of 5 m spatial resolution can be obtained, by including up to 21% interpolated values. Finally, we use ultrafine particle concentrations to consider the minimum number of sampling runs needed to make a representative concentration map with a specific spatial resolution, finding that generally between 15 to 21 repeats of a particular route under similar traffic and meteorological conditions is sufficient. The concentration maps can afford insights into factors influencing pollutant concentrations at the city block and sub-block scale; information that is useful in urban planning strategies to reduce pollution exposure. Methodical analysis of mobile monitoring data will facilitate meaningful comparison of concentration maps of different routes/studies.
Pollutants in tailpipe emissions can be highly elevated around roadways, and in early mornings the pollution plume can extend hundreds of meters into surrounding neighborhoods. Solid sound walls and vegetation barriers are commonly used to mitigate noise, but they also help mitigate near-road air pollution. Here we assess the effectiveness of barriers consisting of vegetation only and of a combination of vegetation and a solid sound wall (combination barrier) in reducing pollution concentrations downwind of roads, under stable atmospheric stability and calm to light wind conditions. Because there was no practical (no barrier) control site in the area, we primarily compare the two barrier types to each other and explore the importance of atmospheric conditions. Using measurements collected with a mobile platform, we develop concentration decay profiles of ultrafine and fine particles, oxides of nitrogen (NO and NO 2) and carbon monoxide downwind of a freeway in California with different barrier configurations and meteorological conditions. Diurnally averaged data collected with passive samplers indicate that pollution from morning rush hour has about equal impact as the entire remainder of the day, because of differences in atmospheric dispersion as the day progresses. Under calm and stable atmospheric conditions (wind speed < 0.6 m/s); a vegetation-only barrier was more effective than a combination barrier with a total height that was somewhat lower than the vegetation-only barrier, by 10-24 % in the first 160 m downwind. Under light winds (above ~ 0.6 but below 3 m/s) and stable conditions, the combination barrier was more effective the vegetation barrier alone, by 6-33%, in the first 160 m from the barrier. The average particle size downwind of the vegetation-only barrier was larger than downwind of the combination barrier, indicating that particle deposition plays an important role in the reductions observed downwind of vegetation. Our results are consistent with the notion that at low wind speeds, vegetation acts as an effective barrier. Overall, adding vegetation alone or to an *
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